ITER Neutral Beam Test Facility

The ITER Neutral Beam Test Facility is a part of the International Thermonuclear Experimental Reactor (ITER) in Padova, Veneto, Italy.^[1] The facility is being built to host the full-scale prototype of the reactor's neutral beam injector, MITICA, and a smaller prototype of its ion source, SPIDER.^[2] SPIDER is planned to start its operation in December 2017. SPIDER will be used to optimize the ion beam source, to optimize the use of cesium vapor, and to verify the uniformity of the extracted ion beam also during long pulses.

To deliver power, two heating neutral beam injectors provide the power of 17MW each to a four-meter diameter container containing burning fusion plasma. This is the main auxiliary heating system of the reactor. Due to its low conversion efficiency, the neutral beam injector first needs to start a precursor negative ion beam of 40A, and then neutralizes it by passing it through a gas cell (<60%), and then by a residual ion dump (the remaining 40% — 20% negative, 20% positive). The neutralized beam is then dumped on a calorimeter during conditioning phases, or coupled with the plasma. Further reionization losses or interception with the mechanical components reduce its current to 17A.^[3]

Purposes

The role of the test facility includes research and development on the following topics:

• voltage holding: due to neutron environment, this will be the first beam source at -1MV with vacuum insulation instead of gas insulation (SF_6 gas is typically used);
• negative ion formation: the requirement on the extracted current density from the cesiated ion source is at the limit of the present technology of plasma ion sources.
• beam optics: the precursor ion beam is generated in a multigrid electrostatic accelerator, having 1280 apertures in each of the 7 grids composing it. Since the overall width of the beam along the beam drift (about 25 meters) is due to the optics of each of the 1280 beamlets, the grid alignment and the disturbances produced by magnetic fields and electrostatic error fields are to be carefully verified.
• vacuum pumps: two 8m long, 1.6m high cryopumps will be installed on each side of the vacuum vessel. The fatigue life of components operating with cycles between 4K and 400K is to be verified.
• heat load on mechanical components: on the electrodes used for beam acceleration, and along the beam path, mechanical components are subject to very high thermal loads. These loads are continuously applied during long pulses, up to 1h. These loads are anyhow lower than the heat loads expected on the ITER divertor plates.

References

External links

• Consorzio RFX website